Cross section calculation question...

[J-Rod]

I have a question, and I have looked over my math, and I don't see any errors. But in looking at the results, I know I have to be wrong somewhere. So, just wanted tu run it by folks to see if I have missed something...

I was calculating intake runner length using both simple area (LxW) and also using the formula for CSA:
Degrees to subtract 20 DGR = Degrees to Remove
ECD 455 Effective cam duration (ECD) = 720 - (Adv. duration - DGR)
optimum intake runner length (L) is:L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Length 4.974882832 L = Optimum Runner Length

Reflective Value 1 RV = Reflective Value (which set of pressure waves do you want to use 1st, 2nd or 3rd, etc…)
Runner Diameter 2.094351984 D = Runner Diameter (Lsx 3.25x1.06=3.445 Area )
RPM 6800 RPM=RPM range you wish to tune port for
Velocity 180 V= Velocity - The velocity in the plenum intake pipe should not be higher than 180 ft/sec at maximum rpm.



3.25 L= Port Length
1.06 W = Port Width
3.445 Area of a rectangle = L x W

Area of a Circle = Pi x r^2
Convert Area of a simple rectangle to a circle
1.096577558 Divide the area (in square units) by Pi (approximately 3.14159).
1.047175992 Take the square root of the result. This is the radius.
2.094351984 Now double the radius to get the diameter.


Intake Runner Length Using Cross Sectional Area
Calculate Cross Section Area
3.25 L= Port Length
1.06 W = Port Width
8.62 Circumference = (L+W+L+W)
2.155 Square = (Circumference/4)
4.644025 CSA = (Square*Square)

Convert CSA to a Circle to obtain diameter
1.478239069 Divide the area (in square units) by Pi (approximately 3.14159).
1.215828553 Take the square root of the result. This is the radius.
2.431657105 Now double the radius to get the diameter.

Degrees to subtract 20
ECD 740 Effective cam duration (ECD) = 720 - (Adv. duration - DGR)
optimum intake runner length (L) is:L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Intake Runner Length 8.578289095 L = Optimum Runner Length

Reflective Value 1 RV = Reflective Value (which set of pressure waves do you want to use 1st, 2nd or 3rd, etc…)
Runner Diameter 2.431657105 D = Runner Diameter Using CSA
RPM 6800 RPM=RPM range you wish to tune port for
Velocity 180 V= Velocity - The velocity in the plenum intake pipe should not be higher than 180 ft/sec at maximum rpm.


Now, the kicker. I tried to calculate peak torque RPM which came up REALLY high.

Peak Torque 9466.63369 peak tq rpm=avg. cross-sectional area of intake manifoldx88200/cubic in. of cyl
For instance: 3.87x88200/50=6800rpm for a typical 400" sbf with a ported super victor manifold.


Anyhow, just wondering if the forumlas I'm using are wrong, or I'm just missing something...

Thanks.

[blownzoom440]

http://www.rbracing-rsr.com/runnertorquecalc.html i am not sure if it will help but here is a calculator for peak TQ.

[J-Rod]

Thanks, they are using the same formula, thus they got the same results. It appears to be based on the Cross sectional area. I'm taking

L+w+L+w= Circ.
Then circ/4 = sq
Then sq*sq

Anyhow, thanks for the link...

[Rick360]

Intake Runner Length Using Cross Sectional Area
Calculate Cross Section Area
3.25 L= Port Length
1.06 W = Port Width
8.62 Circumference = (L+W+L+W)
2.155 Square = (Circumference/4)
4.644025 CSA = (Square*Square)

This is not how to calculate CSA. CSA is the same as the area you calculated in the first equation you call "area of a rectangle". CSA and area of a rectangle are the same thing. Height * Width=CSA of a port. 3.25 * 1.06=3.445sq.in. It will be less by a small amount due to the corner radius. For .250" radius corners subtract .054sq.in.

I'm not sure what you are doing and why in all of your formulas, but this part is not right.

Rick

[OldSStroker]

Thanks, they are using the same formula, thus they got the same results. It appears to be based on the Cross sectional area. I'm taking

L+w+L+w= Circ.
Then circ/4 = sq
Then sq*sq

Anyhow, thanks for the link...

Thoughts:

FWIW, two decimal places is probably sufficient accuracy for port areas, IMO. Anyway...

For a given circumference, a circle gives the most area. For a given circumference rectangle, a square gives the most area.

Examples: A(circle) = PI/4 x dia x dia
PI/4 is very close to .7854 which conveniently the numbers in the upper left corner of a calculator keyboard. A = .7854 x dia x dia

So a 1 inch dia. circle has an circumference of PI x dia. or 3.14159 and an area of PI/4 x 1 x 1 or .785398 or .7854 x 1 x 1 = .7854 sq. in.

The side of a square with a circumference of PI is PI/4 or .7854. The area would be .7854 squared or .7854 x .7854 = .6169 or 78.54% of the circle's area.

A 3.14159 circumference rectangle could have various areas: 1.071 x .500 gives a circumference of 3.142 (close enough) and an area of .5355, smaller still. The skinner the rectangle, the less the area. At .100 wide and 1.5028 long, the rectangle has an area of .152 sq. in. or 19.4% of the circle.

To convert a rectangular area (L x W) to an equivalent circular area of Diamerter (D):

L X W = .7854 x Dx D
solving for D^2 (D squared),

D^2 = (L x W / .7854)

So D = the square root of (the area divided by .7854)

Example. L=3.25 W=1.06

D = (3.25 x 1.06 / .7854)^.5[square root]
D = 2.094, which you got one time.

Also, I don't think a SBC SuperVic average runner cross-section is 3.44 sq inches. Certainly the head end of the runner and the port opening is smaller than that. I get somewhere around about 2.75 sq. in. for a well ported AFR SBC 23* head at the intake flange.


Using 2.75 sq in on a 400 gives about 4850 rpm for peak on the "88200" calculator. This sounds reasonable to me.

My $.02

[J-Rod]

Intake Runner Length Using Cross Sectional Area
Calculate Cross Section Area
3.25 L= Port Length
1.06 W = Port Width
8.62 Circumference = (L+W+L+W)
2.155 Square = (Circumference/4)
4.644025 CSA = (Square*Square)

This is not how to calculate CSA. CSA is the same as the area you calculated in the first equation you call "area of a rectangle". CSA and area of a rectangle are the same thing. Height * Width=CSA of a port. 3.25 * 1.06=3.445sq.in. It will be less by a small amount due to the corner radius. For .250" radius corners subtract .054sq.in.

I'm not sure what you are doing and why in all of your formulas, but this part is not right.

Rick

I originally was using simple LxW, but I was advised this was not correct. Here is the rationale behind it.

CROSS-SECTIONAL-AREA (CSA): Head lingo, that refers to the actual cubic inch (usually) measurement of area for a port in a head, or intake manifold passage (or any other device flowing air, gasses or fluids). Cubic Centimeters (CC's) is the other value of measurement, but rarely used in U.S. development, EXCEPT in the measurement of cylinder head combustion chambers, piston domes and cylinder's inch conversions to CC's with calculating NET static COMPRESSION RATIO. The CSA calculation is NOT derived from a simple width times height formula. It is based from the square root of a value of area. A simple way to get it, which is good for aspiring head developers who slept through all the advanced math classes in school, depends on which way you are going to do the calculation. If you are taking a PORT and trying to find the CSA, you want to turn the shape into a CIRCUMFERENCE dimension; then DIVIDE by FOUR; then MULTIPLY this figure times itself. (i.e., A small block Chevy might have a port that is 1.25" wide, by 2.125" tall. There are TWO sides to EACH of these dimensions. So pretend you are a piece of string wrapping around the inside space of these dimensions and you will have a TOTAL LENGTH (circumference) of 6.75". Now, DIVIDE this by FOUR (4) to get our "square" for a square root multiple, and you have 1.6875". Now multiply this figure times itself (the essence of the square root), and you get: 2.847". That's the Cross-sectional-area. Now what if you are doing the opposite, and taking something that has a ROUND value already, and needing to find out the cross sectional area of it? Lets use the VALVE. Lets say our small block Chevy head has a valve diameter of 2.100". Here you need to use the standard, Pi (3.1416) to change this circumference into our "string" measurement wrapping around it's circumference (not diameter). In this case that would be 2.100" x 3.1416= 6.59736. (To be exact.) Now divide this by four (4), and you get 1.64934. Multiply this times itself (1.64934 X 1.64934) and you get: 2.720". It's easy. All you are doing is converting shapes into a circumference, then dividing that into an even space of four equal parts, then you multiply one of those equal parts times itself. You are creating a four side box, and multiplying the width times the length. But this ONLY works for EQUAL sides of FOUR. That's why if you multiplied the 1.25" X 2.125" of our sample intake port, you'd have come up with 2.656", and this wouldn't have been the actual cross-sectional-area. It was 2.847", as shown above. In case you haven't noticed.

My issue was htat for a stock Ls1 head it shows the torque peak at 9466RPM based on the CSA.

Anyhow, just looking to see if I'm not understanding things, or the port is just that good.

[buddy rawls]

the effective cross-section of a path is what the cylinder sees. This is seen as a restriction to the cylinder, either in expelling or drawing in. the way I calculate cross sectional area is to take the total path volume and divide by the length, plus a metric conversion to get it in terms of inches (or rather sq inches). the long write up above lost me. I will have to read it thru slowly, but it seems like they are taking math liberties at determining a value that is pretty easy to measure and calculate, by conventional methods.

In terms of cylinder heads, it is easiest for me to approximate length as an average of the floor length and roof length.
avg csa= runner volume cc's/ (avg length inches * 16.39)

I dont remember all the specifics of the LS1 heads but arent they around 210CC with an average port length around 5.5", putting CSA around 2.3 sqin.

Granted this is not the full CSA restriction the cylinder sees, but if the intake manifold is not restrictive to the head runner flow, it will be pretty close.

this is also the real way that cylinder heads can be compared on a equal playing field.

[OldSStroker]

Here is the rationale behind it.

CROSS-SECTIONAL-AREA (CSA): Head lingo, that refers to the actual cubic inch (usually) measurement of area for a port in a head, or intake manifold passage (or any other device flowing air, gasses or fluids). Cubic Centimeters (CC's) is the other value of measurement, but rarely used in U.S. development, EXCEPT in the measurement of cylinder head combustion chambers, piston domes and cylinder's inch conversions to CC's with calculating NET static COMPRESSION RATIO. The CSA calculation is NOT derived from a simple width times height formula. It is based from the square root of a value of area. A simple way to get it, which is good for aspiring head developers who slept through all the advanced math classes in school, depends on which way you are going to do the calculation. If you are taking a PORT and trying to find the CSA, you want to turn the shape into a CIRCUMFERENCE dimension; then DIVIDE by FOUR; then MULTIPLY this figure times itself. (i.e., A small block Chevy might have a port that is 1.25" wide, by 2.125" tall. There are TWO sides to EACH of these dimensions. So pretend you are a piece of string wrapping around the inside space of these dimensions and you will have a TOTAL LENGTH (circumference) of 6.75". Now, DIVIDE this by FOUR (4) to get our "square" for a square root multiple, and you have 1.6875". Now multiply this figure times itself (the essence of the square root), and you get: 2.847". That's the Cross-sectional-area. Now what if you are doing the opposite, and taking something that has a ROUND value already, and needing to find out the cross sectional area of it? Lets use the VALVE. Lets say our small block Chevy head has a valve diameter of 2.100". Here you need to use the standard, Pi (3.1416) to change this circumference into our "string" measurement wrapping around it's circumference (not diameter). In this case that would be 2.100" x 3.1416= 6.59736. (To be exact.) Now divide this by four (4), and you get 1.64934. Multiply this times itself (1.64934 X 1.64934) and you get: 2.720". It's easy. All you are doing is converting shapes into a circumference, then dividing that into an even space of four equal parts, then you multiply one of those equal parts times itself. You are creating a four side box, and multiplying the width times the length. But this ONLY works for EQUAL sides of FOUR. That's why if you multiplied the 1.25" X 2.125" of our sample intake port, you'd have come up with 2.656", and this wouldn't have been the actual cross-sectional-area. It was 2.847", as shown above. In case you haven't noticed.

My issue was htat for a stock Ls1 head it shows the torque peak at 9466RPM based on the CSA.

Anyhow, just looking to see if I'm not understanding things, or the port is just that good.

It must be the new math thing. I got a little lost in the "cubic inch measurement of area". I always thought area was in square units (2-dimensional) and volume was in cubic units (3 dimensional). Perhaps I'm missing something.

Maybe it's non-Euclidian.

J-Rod, yep, the LS1 port is good. I haven't seen any peak torque at over 9400 however.

[panic]

Who wrote that, and on what planet?

"If you are taking a PORT and trying to find the CSA, you want to turn the shape into a CIRCUMFERENCE dimension; then DIVIDE by FOUR; then MULTIPLY this figure times itself. (i.e., A small block Chevy might have a port that is 1.25" wide, by 2.125" tall. There are TWO sides to EACH of these dimensions. So pretend you are a piece of string wrapping around the inside space of these dimensions and you will have a TOTAL LENGTH (circumference) of 6.75". Now, DIVIDE this by FOUR (4) to get our "square" for a square root multiple, and you have 1.6875". Now multiply this figure times itself (the essence of the square root), and you get: 2.847". That's the Cross-sectional-area.

That's the mistake. By his "formula" a port .1" wide by 10" tall (actual area = 1") is 25.5025", an error of 2,549%. It makes rectangles the same area as squares with the same circumference - science is set back 10,000 years.

[J-Rod]

Thats why I'm asking...

Here is the page...

http://mid-lift.com/TECH/TECH-Definitions.htm

[OldSStroker]

Thats why I'm asking...

Here is the page...

http://mid-lift.com/TECH/TECH-Definitions.htm

Thanks for the link.

IMO, there is a lot of opinion mixed in with the definitions.

I agree with "O.E.M." definition, at least as to complete vehicles.

The ROD RATIO definition is a tad confusing to me. If I divide the engines's stroke by the rod length, as "explained" by Miller I get 3.48/5.7 = .61 not 1.64 as we normally discuss it. Yeah, I know, he just reversed the terms, but that gets to be confusing if you are using it as a textbook. Which explanations are correct and which are not?

Cubic area? I think I'll pass on adding this to my Favorites.

[buddy rawls]

Where are they taking their cross section measurement. The smallest cross-section is not correct and the largest is incorrect too. I see what they are doing, but they are taking a funky approach at determining a quantity that is pretty meaningless, unless they do that calculation at about every 1/4" of the port length, then how do you measure it when it starts going into the throat region, because you start getting the long wall and short wall situation, where you are effectively are using a sort of pie slice, if viewed from the side of the port.

CSA is in terms of sq inches, volume is in terms of cubic inches, length is in inches. This is definition. How do you determine the CSA of a path? you take the volume and divide by the length. It does not get any simpler than that (period). how do you determine volume, you take the CSA times the length. they are getting all hung up in terms of automotive versus math/engineering. they are the ones that seem to be using the weird terminology and assignments, not the industry. cross-sectional area is just that. there is no such thing as a cubic inch measurment of an area.

For paths that have varying lengths and turns and internal dimensions, the calculated CSA will be a sort of average. determining the length is usually the tent pole. but for a typical head runner, the average of the roof length and the floor length will get you pretty close to the effective length.

Even in pipe flow problems, they do it this way. ITs called an "effective hydraulic area", right in the engineering fluid flow texts.

[panic]

Too rude, not appropriate.

[Rick360]

Anybody can be an expert on the internet!

Don't believe everything you read or hear.


Rick

[panic]

I just spent 10 minutes reading more on that page, and to be fair some of it is not only correct but quite profound, and some is complete crap.
Rather unusual for someone to be on both sides of the competence fence.